Simple +30fps

[Ian_Gere]

Hi, so as some will recall, I've had problems with the fps of my AK. A quick recap for those who don't know the story so far:

Chrono'd @351fps 1st thing in morning

Later that day, drew blood.

Fitted SAT Variable Velocity Device

It didn't work - fps dropped to between 60 - 170

Added tape around the cylinder head of the VVD to force the O-ring against the cylinder

Chrono'd at 339fps dropping a little during the day.

Put the original piston back in with a Dream Army M110 spring

Chrono'd just above 300fps

Went back to the original spring, but the O-ring didn't seem to be working too well...

I supposed that all the messing about with it may have knackered it, or something else that I don't understand.

Thought about adding tape as I had with the VVD, but I had noticed a slight drop in ROF and was also concerned about the motor...

But I had an ICS Turbo 3000 motor that I hadn't got round to trying out yet, so I put that in...

Then a lightbulb went on above my head and so instead of the tape bodge, I added a 2nd O-ring that was not for airsoft, just something I had lying around amongst my tinkering tat. It is smaller and thinner than a proper piston O-ring and I've fitted it behind the original one. It doesn't touch the cylinder, it just forces the main O-ring to stay pressed forward and stay completely round.

It chrono'd yesterday @average 383fps. I think there is a tiny drop in ROF, but it's difficult to tell, so it can't be much if any.

Now that of course was a problem and I had to put too much hop on to get it down to consistently under 370fps, the hard limit at Skirmish. Still, if any of you find your self wallowing in the 310+ doldrum zone or, more likely, the 330+ area and are planning a trip to a woodland site that limits at 350+/- 5%, this may have interest for you.

I'll be having my AK to bits again soon, because I have to swap the gearbox case and I can't leave it with BBs rising at extreme range, so when I do I'll take pics and add them to this thread.

I'm still left with the problem of getting it consistently at 345fps-ish so I can use the same gun for woodland and CQB. Amongst my current different options are a Guarder 50 deg Hop Bucking and what appears to be an ICS Bore Up cylinder set (it's in the gearbox I bought to swap cases with my damaged one so I'm going on the external look of the thing). I'm hoping that the M110 Spring, new hop and bore up together will work out somewhere near my goal. What do you lot reckon?

 

[SAS scott]

Its best just to get rid of the VVD they are not good.. They adjust the FPS by making the airseal better or worse, but you always want the airseal to be as good as possible to make the gun consistent.

 

[Ian_Gere]

Oh yeah, the VVD went as soon as I got home.

I was away for the weekend for 2 days @The Stan. I had to hire the 1st day because the VVD fμ<Жed my gun up. Next morning I stripped it and added the tape as a stop-gap measure. I've had a refund from Airsoft World without any hassle, which is good of them I suppose, because I bought it for £40 half price in one of their 60 minute sales and it wasn't in the same condition I received it in, having been greased and fitted.

The new bodge with the 2nd O-ring is on the original piston and spring set up. I did it because there seemed to be very little compression in the cylinder when I tested it by pushing with my fingers before I rebuilt the gearbox. As it happens, it seems to have worked too well!

 

[SAS scott]

Oh yeah, the VVD went as soon as I got home.
I was away for the weekend for 2 days @The Stan. I had to hire the 1st day because the VVD fμ<Жed my gun up. Next morning I stripped it and added the tape as a stop-gap measure. I've had a refund from Airsoft World without any hassle, which is good of them I suppose, because I bought it for £40 half price in one of their 60 minute sales and it wasn't in the same condition I received it in, having been greased and fitted.

The new bodge with the 2nd O-ring is on the original piston and spring set up. I did it because there seemed to be very little compression in the cylinder when I tested it by pushing with my fingers before I rebuilt the gearbox. As it happens, it seems to have worked too well!

Ahh ok mate sounds like an air leak issue, have you tried heating the piston head o-ring up then stretching it over the cylinder? That can improve airseal also wrapping the cylinder head with PTFE tape will be worth a try (I do it on all my guns).

Bore up kits IMO dont tend to be good (I always have had bad experience) but defiantly fit a new hop rubber to see if that increases the airsoft also do the dental floss mod on the hop rubber.. With good air seal most guns should be around 350 with an M100 spring. I once worked on a gun which was fireing at 230 FPS with an M120 just because of bad airseal between the hop unit, barrel and nozzle.

 

[Ian_Gere]

Well so far I haven't had the hop/barrel interface apart yet so I don't know what that air seal is like, but if there's any gap that PTFE tape can fit in, it will go in!

I have to say that I have my doubts about the air nozzle that came with the gun. It's transparent and I can see 2 tiny cracks in the end, just where it rounds off at the tip. I've got a new one on order, but I've also ended up with one fitted, weirdly enough, inside the gearbox shell and sticking out like the cylinder head tube would if it were done correctly, with the new gearbox I bought for the shell. Having said that, the bore up cylinder is fitted the wrong way so that the tear drop shaped holes are at the cylinder head end as well! I can only hope that whoever last worked on it was making a joke and not just a pillock, otherwise who knows what I'll find when I do get round to opening it up!?!

I'm not sure that I could get any PTFE around the cylinder head and still get it inside the cylinder, but I'll have a go. Do you cover the O-ring/s or put the tape on then the O-ring/s on top?

What is the dental floss mod?

 

[bigshep]

and the moral of the story is, if the gun works leave it the f alone.

My cyma Ak has been downgraded with a m90, fire fine at 320fps, has a nice ROF and works with good range, only thing I MAY do is stick a tightbore or TK twist in it if I can be bothered.

 

[Ian_Gere]

and the moral of the story is, if the gun works leave it the f alone.
My cyma Ak has been downgraded with a m90, fire fine at 320fps, has a nice ROF and works with good range, only thing I MAY do is stick a tightbore or TK twist in it if I can be bothered.

Good point, but I started with a gun that was just too hot for sites that limit to 328fps +/- 5% or 350 max. I enjoy Skirmish immensely, but I want to be able to take my AK all over the place without messing with it constantly.

I realise that some people are of the opinion that 30fps doesn't make enough difference in range to bother about, or that you can get that extra range from a well set up hop unit. My opinion is that I have noticed the difference in range at the various powers that I've had at my disposal, all with the exact same hop up. Therefore, if I do put better hop parts in, I will get even more range with a gun that's just under the 350 limit than the same upgraded hop shooting at, say, 320 like yours.

A tightbore for it is on my medium-term agenda, but I also want to add a silencer as a barrel extension and lengthen the inner barrel to 590mm. Why? For the accuracy at long range! This is my 1st AEG - my learning gun. I want to see what's possible without spending fortunes, so I'm seeing what I can get 2nd hand or from Hong Kong... and also what bodges can achieve.

 

[SAS scott]

I'm not sure that I could get any PTFE around the cylinder head and still get it inside the cylinder, but I'll have a go. Do you cover the O-ring/s or put the tape on then the O-ring/s on top?
What is the dental floss mod?

You rap the tape around the o-rings then grease it all up with silicone grease, this will make it easier to put in and help with airseal. The dental floss mod is where you tightly rap floss around the hop rubber then tie it in a not.. This improves airseal more than just wrapping it with ptfe tape.

Since you have said the nozzle is cracked I would suggest getting an SHS metal nozzle, in my experience they are the best nozzles for airseal.

 

[Ian_Gere]

Yeah, I was sold on the idea of SHS ali nozzles too, until I heard from somebody in a thread I started about BB choice. He had damage to his piston head from fragments of BB ending up inside the cylinder. What could cause that I wondered... I've bought a PSEM Classic Electro nozzle that looks to be made of nylon. I went with that over my other choice which was to just bite the bullet and buy Systema (I mean £9 isn't £100s like most of their stuff, so I was tempted), because it comes with 2 O-rings. Only 1 problem, they are 2 different thicknesses and I don't see any instructions... lol!

Thanks for the other tips. Much appreciated. I may pm you when I get round to doing these mods, if you don't mind?

 

[Ian_Gere]

Ah now, first thing is I must apologise. When I wrote the first post I had forgotten exactly what happened with the second O-ring bodge. Since having it apart, I now remember...

The extra O-ring kept slipping from behind the one the piston head is designed for, so I put it slightly further back, held in place by a very thin acetate washer that was already between the piston and piston head. You can see it here (apologies also for phone pics, but I don't have a digcam):



The O-ring went in like this:



And here it is on the right for comparison to an ordinary piston O-ring: